When trying to define a genre, we must look for characteristics of the genre.

The characteristics of a genre are found in the corpus of works that belong to the genre.

The corpus can only be found by defining the characteristics.

So genre = characteristics = corpus = genre ----> tautology.

Donato Totaro [2000] said almost the same when he stated in relation to the horror genre that:

"Admittingly, I am swimming into a tautological sea by assuming a general understanding of the horror genre. But a certain tautology is a necessary evil of genre theory, and I mean the latter more as a categorization rather than definition of the horror genre. Besides, I do agree with Noel Carroll's other not sufficient descriptions of "art-horror" (the quality of being "interstitial," ie. something that can not be categorized, as for example, vampires, mummies, zombies that are neither living nor dead; impurity, disgust, etc.)." --Donato Totaro, 2000 via [1]

"To take a genre such as a ‘Western’, analyse it, and list its principal characteristics, is to beg the question that we must first isolate the body of films which are ‘Westerns’. But they can only be isolated on the basis of the ‘principal characteristics’ which can only be discovered from the films themselves after they have been isolated. That is, we are caught in a circle that first requires that the films be isolated, for which purposes a criterion is necessary, but the criterion is, in turn, meant to emerge from the empirically established common characteristics of the films." --Theories of Film (1974), chapter " Critical Method: Auteur and Genre", Andrew Tudor

But before Totaro, Sconce and Tudor, there was Damon Knight [1952] who said: